--> ABSTRACT: Carbonate Pore Type Identification Using Fuzzy Logic and Open-Hole Logs; Case of Study: Cretaceous Formation in Lake Maracaibo, by Soto, Rodolfo; Arteaga, Duarry; Martin, Cintia; Rodriguez, Freddy; #90135 (2011)

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Carbonate Pore Type Identification Using Fuzzy Logic and Open-Hole Logs; Case of Study: Cretaceous Formation in Lake Maracaibo

Soto, Rodolfo 1; Arteaga, Duarry 2; Martin, Cintia 2; Rodriguez, Freddy 2
(1)DIGITOIL, Bogota, Colombia. (2) PDVSA, Maracaibo, Venezuela.

Today, natural fracture and/or connected vuggy systems in carbonate reservoirs contribute significantly to hydrocarbon production. If a carbonate reservoir has only intergranular porosity, the water saturation, Sw, can be calculated with high certainty using the Archie equation with a cementation factor, m, equal 2. But it is common to find in a carbonate reservoir a complex pore system that may be intergranular, vuggy, and fractured. If we use m equal 2 for vuggy or fractured porosities, the water saturation will be too low or too high respectively, leading engineers to easily make some incorrect decisions.

Despite its importance, one of the difficult tasks during the petrophysical evaluation of carbonates is to identify the multipore system present in each case. Engineers use data from cores, well-logs, and pressure transient tests to apply different techniques for this purpose. Unfortunately, core and pressure analyses are not always available and most of the techniques from well log analysis published in the literature do not manage imprecise data or factors that distort the data because of the pore complex systems in carbonates.

We present in this paper a new methodology for the reduction of uncertainty in pore type identification and characterization using soft computing technology with conventional well-log data. Membership functions were built for each conventional well log. Their outputs indicated the probability of fracture/vuggy/intercrystalline pores according to the logs analyzed. For each function, we determined a weight and a threshold parameter adjusted from core data and/or borehole images. Then we calculated the fracture or vuggy porosity from a resistivity-porosity model using the m variable determined from Pickett plots for each pore type. We found for our case that m=2.46, 2.0 and 1.4 for vuggy, intercrystalline, and fractured formations respectively. The total and effective porosities were calculated from neutron-density-sonic logs.

Finally, we calculated Sw for each well and found that our models adjust with field results, explaining the behavior of each well with high certainty from the beginning of production. We have successfully applied our methodology to identify the complex pore types for the Cretaceous formation in Lagomar and Lagomedio fields in Venezuela.




 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.